A touch sensor (“touch panel”) is an input device which performs position detection as to a place where contact with a finger, a pen, or the like is made, and conveys the intent of an operator to an information processing system. Known approaches of position detection are a static capacitive coupling approach, a resistive membrane approach, an infrared approach, an ultrasonic approach, an electromagnetic induction/coupling approach, and so on.
A touch sensor of the static capacitive coupling approach is disclosed in Patent Document 1, for example. A touch sensor of the static capacitive coupling approach is constructed so that terminals for position detection are formed in the four corners of a conductive film for position detection, and an AC voltage is applied to the terminals. When a contact point is formed with a finger, a pen, or the like on the conductive film for position detection, the transparent conductive film for position detection is coupled to the ground (ground plane) in a capacitive manner. By detecting values of currents flowing between the contacted portion that has established capacitive coupling and the terminals at the four corners of the transparent conductive film for position detection, the position coordinates of the contacted portion are ascertained.
In the case where a touch sensor is used in an integral form with a display device, for example, the touch sensor is to be disposed on the front face (viewer side) of a display panel such as a liquid crystal panel. In this case, there is a problem in that the touch sensor receives noise from the display panel, thus degrading the accuracy of position detection of the touch sensor. Noise from the display panel includes, for example, an induced voltage that occurs on the transparent conductive film for position detection, which is comprised in the touch sensor, responsive to a common voltage that is applied to a counter electrode, which is comprised in the liquid crystal panel.
Conventionally, in the case where a touch sensor of the static capacitive coupling approach is disposed on a liquid crystal panel, a shield layer is placed between the transparent conductive film for position detection (which is comprised in the touch sensor) and the liquid crystal panel, the shield layer preventing the touch sensor from being unfavorably affected by noise from the liquid crystal panel. Furthermore, the transparent conductive film for position detection of the touch sensor is disposed sufficiently far from the liquid crystal panel, which also suppresses the influence of noise from the liquid crystal panel.
A display device which comprises a touch sensor of the resistive membrane approach is disclosed in Patent Document 2, for example. In this publication, the timing with which output data is taken from the touch sensor is varied to ensure that noise originating in the liquid crystal panel will not be mixed in the output data.
Moreover, as a method for avoiding noise ascribable to the display signal, a method is also being practiced in which the position detection of a place of contact is performed during a blanking period, in which the display signal is absent.
On the other hand, the applicant discloses in Patent Document 3 a display device and a position detection method comprising a touch sensor of the static capacitive coupling approach, which takes advantage of a voltage which is induced in the conductive film for position detection by a voltage that is supplied to the display panel. When this construction is adopted, there is no need to provide a shield layer, so that parallax can be made smaller than conventionally.
Moreover, Patent Document 4 discloses a coordinate detection device which, in order to suppress noise influence, sets an AC voltage to be applied to the conductive film for position detection to a frequency between an nth order harmonic and an (n+1)th order harmonic of the induced voltage. Patent Document 5 discloses a coordinate input device which, in order to suppress noise influence, sets an AC voltage to be applied to the conductive film for position detection to a period in the neighborhood of 1/n of the period of the induced voltage.    [Patent Document 1] Japanese National Patent Publication No. 56-500230    [Patent Document 2] Japanese Laid-Open Patent Publication No. 9-128146    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2004-21327    [Patent Document 4] Japanese Laid-Open Patent Publication No. 62-037725    [Patent Document 5] Japanese Laid-Open Patent Publication No. 62-165229